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Abstract:

A method for protecting silver and silver alloy surfaces against
tarnishing, characterized by initially subjecting the surface to be
treated to cleaning pre-treatment in organic solvents; immersing the
cleaned surface in an acid solution able to ensure formation of a thin
layer of silver oxide; immersing the oxidized surface in a solution of at
least one thiol of formula CH3(CH2)nSH where n is between
10 and 16; and chemically reacting the molecules of said thiol with the
previously oxidized silver surface, in an environment containing water
vapor at a temperature of at least 50° C.

Claims:

1. A method for protecting silver and silver alloy surfaces against
tarnishing, comprising: initially subjecting a surface to be treated to
cleaning pre-treatment in organic solvents; immersing the cleaned surface
in an acid solution able to ensure formation of a thin layer of silver
oxide; immersing the oxidized surface in a solution of at least one thiol
of formula CH3(CH2)nSH where n is between 10 and 16; and
chemically reacting molecules of said thiol with the oxidized surface, in
an environment containing water vapor at a temperature of at least
50.degree. C. for at least ten minutes.

2. The method as claimed in claim 1, wherein the step of initially
subjecting a surface to be treated to cleaning pre-treatment comprises
cleaning the surface to be treated by immersion in at least one organic
solvent.

3. The method as claimed in claim 2, further comprising heating the
surface immersed in the at least one organic solvent.

4. The method as claimed in claim 3, further comprising subjecting the
surface to thermal agitation during the heating.

5. The method as claimed in wherein the step of immersing the cleaned
surface in an acid solution comprises immersing the cleaned surface in a
sulphuric acid solution.

6. The method as claimed in claim 1, wherein the step of immersing the
oxidized surface in a solution comprises immersing the oxidized surface
in a pentadecanethiol solution.

7. The method as claimed in claim 1, wherein the step of immersing the
oxidized surface in a solution comprises immersing the oxidized surface
in a hexadecanethiol solution.

8. The method as claimed in claim 1, wherein the step of immersing the
oxidized surface in a solution comprises immersing the oxidized surface
in an undecanethiol solution.

9. The method as claimed in claim 1, wherein the step of immersing the
oxidized surface in a solution comprises immersing the oxidized surface
in a solution of at least one thiol at a temperature of about 30.degree.
C. for at least two hours.

10. The method as claimed in claim 1, wherein the step of chemically
reacting molecules of said thiol with the oxidized silver surface
comprises chemically reacting the molecules of said thiol with the
oxidized surface, in an environment containing water vapor at a
temperature of at least 50.degree. C.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to a method for protecting silver and
silver alloy surfaces against tarnishing.

BACKGROUND OF THE INVENTION

[0002] Spontaneous tarnishing of silver and silver alloy surfaces exposed
to the atmosphere is known to constitute a serious problem for silverware
producers. In this respect it has been shown by various market
investigations that one of the main reasons for the reduction in the
market for silverware articles is linked to the tarnishing of their
surfaces as a result of the formation of sulphurated silver compounds.

[0003] The tarnishing of silver articles, where the term "silver articles"
means both solid silver articles and articles made of base metals or
alloys coated with silver layers of micrometric thickness, is a process
involving only the metal surface, without resulting in deep corrosion of
the metal itself. It involves progressive alteration of its reflectance
spectrum, giving rise to a variation in the surface colour. Although the
process does not produce irreversible damage, silver tarnishing requires
methodical surface cleaning which, although irksome for the final user,
is even more so for distributors and retail sellers.

[0004] For this reason, the protection of silver surfaces against
tarnishing has been the subject of much research over the years. In
particular it has already been proposed to prevent or significantly
reduce tarnishing by protecting for example the silver surface by thin
polymer layers or by using waxes or surfactants which are deposited on
the article surface on termination of its production. These known methods
are based on the principle of preventing or generally hindering the
adsorption of oxygen, of sulphurated volatile oxidants such as SO3,
or of non-oxidizing sulphurated volatile compounds such as H2S, onto
the metal surface.

[0005] Methods are also known which, both in the case of solid articles
and of articles made of base metal alloys coated with the noble metal,
prevent surface tarnishing by replacing the silver with alloys of noble
metals which are less electropositive and hence less sensitive to
tarnishing processes.

[0006] However all these known methods present the substantial drawback of
perceptibly modifying the article appearance as they necessarily involve
a variation in the material reflectance spectrum.

[0007] Methods have also been proposed for depositing spontaneously
ordered molecular layers of organic molecules (known as spontaneously
assembled [SAM] layers) on metals such as gold, silver and copper. These
have opened new facilities for the development of processes able to
reconcile the requirement of preventing or delaying surface tarnishing
with the need not to appreciably alter colour and brightness.

[0008] SAM layers are monolayers or sub-monolayers of ordered organic
molecules formed by molecule chemisorption on the surface of solids. In
the case of surfaces of metals of group 11 (IB) of the periodic table
(Cu, Ag, Au), processes are known [J. C. Love, L. A. Estroff, J. K.
Kriebel, R. G. Nuzzo and G. M. Whitesides, "Self-assembled monolayers of
thiolates on metals as a form of nanotechnology" Chemical Review, 105
(2005) 1103-1169] for forming said SAM layers starting from thiols in
solution via the formation of direct metal-sulphur bonds starting from a
suitable solution. The spontaneous formation of self-assembled layers is
of interest according to the present invention, given that these layers
are able to cover surfaces of arbitrary shape and size and can be
obtained without recourse to ultra-high vacuum techniques, with
consequent production cost reduction.

[0009] Moreover it is known that SAM layers with coating grades close to
unity constitute an effective diffusive barrier to sulphurated volatile
oxidant gases such as SO3 and/or non-oxidizing sulphurated volatile
compounds such as H2S, towards the silver surface--and hence their
relative adsorption on the same surface. In particular, it has been
proposed (PCT/US1999/006775) to deposit thiols of general formula
CH3(CH2)nSH on silver surfaces using procedures consisting
of immersing said surfaces in suitable aqueous or organic thiol
solutions. In particular, it has been noted that this procedure is
effective in retarding silver oxysulphide growth and hence in partially
preventing tarnishing of silver articles.

[0010] However this known process is not ideal in preventing tarnishing of
articles subject to ordinary handling or even only to moderate functional
use, if their surface is exposed to minimal abrasion (for example dust
removal by soft cloths) or if their surfaces are brought into contact
with hot liquids. The degraded protection capacity of the surface of
these articles is probably due to two factors:

[0011] the assembly procedure described in WO1999048682 assumes that the
formation of the prescribed covalent bonds between the surface silver
atoms and the thiol sulphur atoms occurs spontaneously according to the
reaction

Ag+RSH→Ag--SR+1/2H2

on the non-oxidized surface (where R is a generic alkyl); i.e.

Ag--O1/2+RSH→Ag--SR+1/2H2O

on the oxidized surface; i.e.

Ag--OH+RSH→Ag--SR+H2O

on the hydroxylated surface. However these reactions do not come to
completion at ambient temperature, and consequently parts of the surface
may not be coated, that is coated in a faint way from physisorbed thiols,
making triggering of local oxidation processes (pitting) possible;
moreover a fraction of the thiols present on the surface could be
physisorbed and not chemisorbed, and hence be easily removed from the
surface by mechanical or chemical means even under mild treatment
conditions;

[0012] on surfaces of articles made of solid silver or of base metal or
metal alloys coated with silver layers, the presence of micrometric width
scoring means that the metal surface is not entirely wetted by the thiol
solution and hence does not react completely with the thiols. This makes
triggering of oxidative processes possible in the unprotected surface
regions, and can consequently explain the appearance of surface
tarnishing (typically not uniform).

SUMMARY OF THE INVENTION

[0013] The problem is solved according to the invention by a method for
protecting silver and silver alloy surfaces from tarnishing, comprising:

[0014] initially subjecting the surface to be treated to cleaning
pre-treatment in organic solvents,

[0015] immersing the cleaned surface in an acid solution able to ensure
formation of a thin layer of silver oxide,

[0016] immersing the oxidized surface in a solution of at least one thiol
of formula

CH3(CH2)nSH

where n is between 10 and 16, and

[0017] chemically reacting the molecules of said thiol with the previously
oxidized silver surface, in an environment containing water vapor at a
temperature of at least 50° C. for at least ten minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A preferred embodiment of the present invention is further
clarified hereinafter by way of non-limiting example with reference to
the accompanying drawings, in which:

[0020]FIG. 2 shows in graphic form the reflection spectrogram of the
visible-ultraviolet region for silver and tarnishing silver surfaces,

[0021]FIG. 3 shows the time variation of the degree of tarnishing
determined in accordance with UNI EN ISO 4538 for silver coated surfaces
treated in accordance with WO1999048682 and in accordance with the
present invention,

[0022]FIG. 4 shows the time variation of the degree of tarnishing
determined in accordance with UNI EN ISO 4538 for untreated silver coated
surfaces and in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0023] As stated, the method of the invention consists of firstly
subjecting the silver coated surface to complete evaluation, removing any
coating of hydrophobic substances and any machining residues present on
the metal surface.

[0024] The surface is degreased by an initial preliminary surface wash
with running water and surfactant.

[0025] After this preliminary step the article with its surface treated in
this manner is immersed in different organic solvents and then heated and
subjected to ultrasonic agitation.

[0026] At the end of this preliminary phase, the article presenting a
surface that has been degreased in organic solvents is immersed in an
acid solution able to ensure the formation of a thin layer of silver
oxide.

[0027] The following represents an example of the implementation of this
pre-treatment:

[0029] a further 10 min in trichloroethylene (or in cyclohexane) under
ultrasonic agitation at 60° C.,

[0030] 10 min in acetone at 60° C.,

[0031] a further 10 min in acetone under ultrasonic agitation at
60° C.,

[0032] 10 min in ethanol at 70° C.,

[0033] a further 10 min in ethanol under ultrasonic agitation at
60° C.

[0034] On termination of the degreasing sequence in organic solvents the
article is immersed at ambient temperature in a 10 vol % sulphuric acid
(H2SO4) solution for 150 seconds and rinsed with bidistilled
water. The purpose of this treatment is to activate the silver surface,
giving rise to a thin oxide layer, possibly hydrated, which facilitates
the subsequent thiol bonding process.

[0035] The article is then rinsed in bidistilled water and dried in a
stream of dry nitrogen.

[0036] On termination of the surface preparation sequence, the article is
immediately transferred into a solution of a thiol of formula
CH3(CH2)nSH where n is between 10 and 16. Preferably a
0.15 M solution of pentadecanethiol (CH3(CH2)14SH) or
hexadecanethiol (CH3(CH2)15SH or undecanethiol
(CH3(CH2)10SH) in isopropanol is used at a temperature of
about 30° C. The article is then left to react for at least 2
hours under magnetic agitation. After this, the sample is initially
rinsed in isopropanol under ultrasonic agitation at ambient temperature
for 10 minutes followed by a second rinse in fresh isopropanol again
under ultrasonic agitation at ambient temperature for 10 minutes. Both
the isopropanol used to prepare the thiol solution and that used in the
wash processes are previously degassed by passing dry nitrogen through
for one hour. The double rinse in isopropanol removes from the surface
those molecules not directly physisorbed on the metal surface. The
article is then dried in a nitrogen stream and placed in an oven in air
at a temperature of 50° C. for at least 10 minutes. This passage
is fundamental in enabling the reaction of eliminating a water molecule
by reaction between the thiol and the oxidized silver surface to go to
conclusion, consequently ensuring virtually complete protection of the
article surface against tarnishing.

[0037] To verify the effective surface protection following the use of the
method according to the invention, a test chamber was fitted out to
evaluate atmospheric tarnishing under accelerated conditions in
accordance with the teachings of Italian Standard UNI EN ISO4538
("thioacetamide corrosion test", March 1998).

[0038] The chamber scheme is shown in FIG. 1. It comprises a test
environment consisting essentially of a glass container 2 with lid 4,
which must be able to resist corrosion by volatile sulphides without
emitting any gas or vapor which can influence the test; a disc 6 of
non-metallic inert material, inserted into the test chamber, which acts a
container for the thioacetamide (CH3)(CS(NH2); and a support of
non-metallic inert material positioned in the test environment above the
disc 6, to support the test pieces 10.

[0039] While conducting the test, the test environment is covered by
absorbent paper 12 well pressed against the vertical walls and soaked in
a solution of sodium acetate trihydrate Na(CH3COO).3H2O (30 g
of sodium acetate trihydrate dissolved in 10 ml of bidistilled water).

[0040] A thin uniform layer of thioacetamide powder 14 is sprinkled on the
disc 6 (in a quantity of about 0.020 g--such as to ensure a degree of
covering of the disc equal to or greater than 0.050 g/dm2). The test
pieces 10 are then placed on the support 8 in a position such as not to
touch either the absorbent paper 12 soaked with sodium acetate or the
thioacetamide on the support disc. The chamber is then temperature
controlled between 25 and 30° C. Under these conditions, the
sodium acetate solution determines at equilibrium a known fixed relative
humidity in the chamber of 75%.

[0041] The tarnishing test is carried out by comparatively exposing test
pieces consisting of a flat plate of base alloy coated with a 7 micron
layer of silver, they having been either treated or not treated by the
method of the invention. The degree of tarnishing is then evaluated after
predetermined time intervals, by comparing the appearance of the test
pieces both visually and by specular reflectance measurements in the
visible-ultraviolet region.

[0042] The spectrograms of a test piece just formed and of a test piece
which has undergone a heavy tarnishing process are shown in FIG. 2.

[0043] In order to verify and further highlight how assembling the thiol
molecule on the surface by chemical reaction with the surface previously
oxidized in an environment containing water vapor constitutes a critical
modification for the purposes of the effectiveness of the surface
protection process compared with that claimed in PCT/US1999/006775 and in
the scientific literature. FIG. 3 shows the tarnishing curve against time
resulting from an accelerated tarnishing test in accordance with ISO 4538
for two silver samples. Of these, the first sample was treated in
accordance with the teachings of PCT/US1999/006775 (hence without
thermally treating the silver surface on which the thiol molecules were
adsorbed), and the second sample was treated by the procedure according
to the present invention and comprising thermally treating the silver
surface at 50° C. in air for 10 minutes after adsorbing the thiol
molecules from solution. Curve A shows the variation in the wavelength at
the deflection point against the treatment time of the silver sample
treated in accordance with patent application PCT/US1999/006775, while
curve B shows the same variation for the silver sample treated in
accordance with the present invention. The comparison between the
spectrograms in the visible-ultraviolet region in specular reflectance
was conducted such as to verify the time displacement of the reflectance
curve deflection point, which in metal silver is positioned at 318 nm and
is displaced to higher wavelengths following tarnishing. The difference
in the protection duration highlights the superior stability of the thiol
layers made to react chemically with the surface following thermal
treatment compared with those obtained in the absence of thermal
treatment.

[0044] For the further purpose of determining the acceleration factors of
the conducted test, untreated silver samples left exposed to the
atmosphere for one year were compared with untreated silver samples
subjected to the accelerated tarnishing test in accordance with ISO 4538.
For the comparison, the aforesaid method of displacing the inflection
point of the reflectance curve was used.

[0045] It was hence determined that the ratio between the first order
kinetic constants of the spontaneous and accelerated tarnishing processes
is equal to about 14000. On the basis of this value it was hence
estimated that the treatment according to the invention can prevent
visually apparent tarnishing for a time exceeding five years.

[0046] The treatment life span was further verified by subjecting samples
treated and samples not treated by the method of the invention to
handling by the bare hands, to mechanical cleaning by soft cloths, by
manual washing with liquid detergent, by dishwasher washing and by
contact with hot liquid foods.

[0047]FIG. 4 comparatively shows the pattern of the tarnishing curves for
the accelerated tarnishing tests carried out in accordance with ISO 4538.
In all the considered cases, the method of the invention was shown to be
able to guarantee acceptable protection for times exceeding four years.

[0048] It was also verified by direct metallographic optical microscope
observation that the method of the invention is also effective on silver
coated surfaces characterized by abrasions deriving from imperfect
polishing of the base metal alloy support.